The present invention relates to disk units having a head, the positioning of which is controlled by use of a burst pattern included in servo data, and a manufacturing method thereof, and more particularly to a disk unit that is devised to avoid an error such as offtrack caused by an abnormal condition of the track width, and a manufacturing method thereof.
Devices using various types of media such as optical disks and magnetic tapes are known as data storage devices. Among them, hard disk drives (HDDs) have become popular as storage devices for computers to such an extent that they are one of the storage devices indispensable for today's computer systems. Further, not limited to computers, their application is widening more and more due to the superior characteristics with the advent of moving picture recording/reproducing devices, car navigation systems, removable memories for digital cameras and so on.
In general, a HDD comprises a magnetic disk as a medium. A track of the magnetic disk is formed by servo data that is written on the magnetic disk by a servo writer, or the like. To be more specific, servo data as shown in FIG. 12(a) is written to follow a concentric track. The servo data is written to a plurality of positions on a track. The servo data is formed of well-known areas including: a Sync part D1 to which Sync data used for data synchronization is written; a STM (Servo track mark) part D2 to which a servo mark indicating the start of the servo data is written; a track ID part D3 having position information indicating a position of a track in a sequence, and the like; and a Burst part D4 to which a burst pattern used for fine position control is written. The Sync part D1 includes servo AGC (Automatic Gain Control) that adjusts an amplification factor of a signal amplifier to keep the amplitude constant before reading out servo data.
For example, as shown in FIG. 12(b), the burst pattern which is written to the Burst part D4 is formed of four kinds of burst patterns, that is, bursts A, B, C, D. Those burst patterns are read by the head, and then the change in amplitude obtained from the read signal (burst signal) is digitized. The digitized change is used for tracking control (track following) of the head element 112, or the like.
In FIG. 12(b), on the assumptions that reference character Tc is a track center, Tb is a track boundary, and Tw is a track width, the bursts A, B are signals alternately written at constant intervals to an area from the track center Tc to one adjacent track center Tc and an area from the track center Tc to the other adjacent track center Tc respectively. In other words, the bursts A, B are signals written to one track boundary Th of one track, and to the other track boundary Th of the one track, respectively. In addition, the bursts C, D are signals written in one track at constant intervals. One of them is written to an odd number track, and the other is written to an even number track. The positioning control of a read head 300a is performed so that the read head 300a is positioned, for example, to the track center Tc that is a position at which the read-signal amplitude of the burst A and that of the burst B are in balance with each other. The discrepancy in position of the head can be detected, for example, by an error signal that is generated in response to the sum of, or the difference between, values obtained by integrating absolute values of the amplitude or of waveforms, which can be obtained from read signals of the bursts A, B.
Methods for writing the burst patterns are two types, which will be described below. To be more specific, a method in which one burst pattern (e.g., burst A) is overwritten (or trimmed) by writing twice; and a method in which one burst pattern is written by writing once without overwriting (or without trimming). If one burst pattern is written by overwriting twice, a seamed part is formed in the one burst pattern. Therefore, this burst-pattern writing method by use of overwriting is called a seamed method in this specification. In addition, a servo pattern written by the seamed method is called a seamed servo pattern. Moreover, if a burst pattern is written without overwriting (without trimming), one burst pattern itself is written by one write operation. Accordingly, there is formed no seamed part. In this specification, this burst-pattern writing method without trimming is called a seamless method, and a servo pattern written by the seamless method is called a seamless servo pattern.
The servo-pattern writing method according to the seamed method and that according to the seamless method will be specifically described below. FIGS. 13 and 5 are schematic diagrams illustrating a servo-pattern writing method according to the seamed method and that according to the seamless method respectively. Servo data is written while a head is moved in the radial direction by a half track. In addition, the burst pattern has bursts A, B, C, D that are written at writing positions differing one after another in the direction in which a track extends.
As shown in FIG. 13, as for the seamed method, for example, in the n-th write operation in a servo track write (STW) process, a write head 300b first writes an ID indicating position information about a position of a track in a sequence, and the like, and then writes only the bursts A, C. To be more specific, a burst pattern is not written at positions of the burst B and D. At the positions of the burst B and D where no signal is written, if a signal has been written by the last servo write, the signal is erased (trimmed). Next, the write head 300b is moved only by a half track. Then, an ID is written, and subsequently the bursts B, C are written. At this time, as is the case with the above, data is erased (trimmed) at specified positions (here, at the positions of the bursts A and D).
Thus, one track is formed by the n-th and (n+1)-th servo writes. Because the track width Tw is usually wider than the width of the read head 300b, a burst pattern is written by overwriting and trimming in this manner. In this seamed method, overwriting servo data at the time of the (n+1)-th write operation to servo data at the time of the n-th write operation makes it possible to write a burst pattern having no gap between tracks adjacent to each other. Here, the bursts A through D are ideally written with their write positions being displaced from one another by a half track in the radial direction. The track center Tc is a position at which when reading the bursts A and B the read-signal amplitude of them are in balance with each other. The distance between the track centers Tc is a track pitch. In addition, a centerline of the two adjacent track centers Tc in the radial direction is a track boundary Tb, and the distance between the track boundaries Tb which are adjacent to each other is the track width Tw. As shown in FIG. 13, the track width Tw is ideally the same as the width of the bursts A through D.
On the other hand, as for the seamless method, as shown in FIG. 5, as is the case with the seamed method, a servo signal is written while a head is moved by a half track. However, one kind of burst pattern is written by one write operation. Since overwrite (trimming) is not performed, unlike the seamed method, for example, at the time of the n-th write (writing the burst A), the burst pattern written by the last write operation is not erased, and accordingly the burst pattern is not trimmed. Accordingly, a gap G is formed between the burst A and the burst B that have been written by the n-th and (n+2)-th write operations. To be more specific, since the width of a write head is usually narrower than the track width or the track pitch, unlike the seamed burst pattern, the width of each of the bursts A through D becomes narrower than the track width or the track pitch. As shown in FIG. 5(e), the track center Tc can be defined as, for example, a position at which the signal amplitude of the burst A and that of the burst B are in balance with each other.
As described above, in the servo track write process, servo data as track information is written so that the servo data can be used for the positioning control of the head. However, this track information may indicate the track width narrowing or widening depending on a position on the disk. In particular, in a hard disk in which TPI (tracks per inch) is increased as a result of the recent increase in recording density, a slight displacement of the track width causes a data position accessed by the head to be incorrect when the head writes data to a track or when the head reads data from the track, and consequently an error occurs.
Heretofore, such a track having an abnormal track width is detected in advance by a test process performed after a STW process. Then, the use of the detected track is disallowed. In order to achieve this, there is, for example, the following method: seeking to all tracks and temporarily writing data to the tracks; and reading out the written data to check whether or not an error occurs.
In addition, as another method, patent document 1 (Japanese Patent Laid-open No. 2003-331545) discloses the off-track avoiding method for avoiding an off-track phenomenon that is caused by nonuniformity in the track width written on a hard disk. As for the technology described in patent document 1, whether or not the track width is normal is judged by use of signals of A, B, C, and D burst areas included in each servo sector. If it is judged that the track width is abnormal, a servo signal of a track having the nonuniform width is erased, and a servo signal is then written by a servo writer again to form a new track.
The judgment as to whether or not the track width is normal is made by: reading a signal of a burst area twice in an off-track state; determining a first added value by adding, for example, a first absolute value T1, which is obtained by subtracting a signal of the burst D from that of the burst B, to a second absolute value T2, which is obtained by subtracting a signal of the burst D from that of the burst A; and checking whether or not the difference between the first added value and a specified value is within a constant range. This method will be further detailed below.
FIG. 14 is a schematic diagram illustrating a method for judging whether or not the track width is normal as described in patent document 1. In the track-width judgment method described in patent document 1, for example, if a track is an even number track at a track center position P101, a read head 300a is moved to P102, P103 that are respective positions moved from the track center position P101 by ¼ track in the radial directions (track width directions) opposite to each other (¼ off-track positions). Then, at the ¼ off-track position P102, an absolute value 332 of the difference between the burst A and the burst C is determined from the burst pattern that has been read out. Further, at the ¼ off-track position P103, an absolute value 333 of the difference between the burst B and the burst C is determined from the burst pattern that has been read out.
In addition, if a track is an odd number track, the read head 300a is moved to P105, P106 that are respective positions moved from a track center position P104 by ¼ track in the radial directions opposite to each other. Then, at the ¼ off-track position P105, an absolute value 335 of the difference between the burst B and the burst D is determined from the burst pattern that has been read out. Further, at the ¼ off-track position P106, an absolute value 333 of the difference between the burst A and the burst D is determined from the burst pattern that has been read out.
In the case of an even number track, the sum of the absolute value 332 and the absolute value 333 is determined; and in the case of an odd number track, the sum of the absolute value 335 and the absolute value 336 is determined. Each sum is then compared with a specified value. If the sum is larger than the specified value, it is judged that the width of the servo sector has increased; and if the sum is smaller than the specified value, it is judged that the width of the servo sector has decreased.